With the rapid development of the steel and iron industry, the shortcomings of the blast furnace ironmaking processes such as heavy dependence on metallurgical coke, high requirements on pellets for use, high energy consumption, and severe pollution become more and more prominent, and the direct reduction ironmaking processes without need of metallurgical coke therefore draw more and more attention. The global production of directly reduced iron reached 73.32 million tons in 2011, and with the continuous development of short-flow processes and electrical furnace steelmaking processes in the global iron and steel industry, robust demands for directly reduced iron, a high-quality steelmaking raw material, will further push its production. The direct-reduction ironmaking technology has become one of the important development directions for new-generation steel and iron technologies.
At present, there are many direct-reduction steelmaking methods proposed both domestically and abroad, which can be classified according to the types of reduction reactors into direct reduction processes using rotary kiln (e.g., the SL/RN method, the Chinese patent application 200710138915.9), rotary furnace (e.g., the Chinese patent applications 200810302482.0 and 201110236682.2), shaft furnace (e.g., the Midrex method, the HYL III method, the Chinese patent application 20111044319.4) and fluidized bed (e.g., the FIOR method, the FIMET method, the Carbide method, the HIB method, the Chinese patent application 201110006745.5), etc.
As compared to other processes, fluidized direct reduction has the following obvious advantages. (1) It can be used to directly process iron ore concentrate powder material, thus eliminating the sintering or pelletizing process. Moreover, it shows ever more prominent advantages in processing mineral powders, since with the continuous exploration of iron ore resources, finer and finer iron ore concentrates will be obtained through grinding and selection of lean ores. (2) Gas-based fluidized reduction of ore fines having relatively large specific surface areas realizes reduction at a low temperature due to low mass transfer resistance and high heat transfer efficiency, and is expected to break through the bottlenecks of conventional direct reduction in efficiency and cost. (3) It can be used to treat complex paragenic ores. For example, fluidized direct reduction-melting separation of vanadium-titanium magnetite concentrate can produce iron, and concentrate vanadium and titanium in the residue.
However, since direct reduction of iron ore concentrate consumes a large amount of reducing gas, and since the reduction reaction is an endothermic reaction, sufficient heat needs to be provided for the fluidized reduction process, besides heating the material to a high temperature, so that the reaction can be performed smoothly. According to relevant industrial test data, the energy consumption for producing hot briquetted iron by the FIOR/FINMET method is about 15.0 GJ per ton, which is much higher than 10.5 GJ of the MIDREX method based on the shaft furnace, suggesting that the processes based on fluidized beds still have much room for improvement in reducing energy consumption. Therefore, how to reasonably improve heat utilization and reduction efficiency is the key to realize large-scale industrial applications of fluidized direct reduction of iron ore concentrate.
The existing processes usually use 2- to 4-stage fluidized beds to conduct fluidized direct reduction of iron ore concentrate. In the process flows of the FIOR method (e.g., the U.S. Pat. No. 5,082,251), the FIMET method (Gerhard Deimek, Steel, 2000, 12: 13-15) and the FINEX method (Shourong Zhang, and Shaoxian Zhang, Steel, 2009, 5: 1-5), four-stage fluidized beds are used. Mineral powder is put into the first stage fluidized bed without preheating, and the preheated reducing gas enters from the last stage fluidized bed and passes through the multi-stage fluidized beds in series. Since there is no intermediary heat supplementation, the temperature declines to below 600° C. at the first and second stages of fluidized beds, at which temperature the reduction rate of the mineral is rather slow and thus the mineral powder is mainly preheated. The reduction of the mineral powder mainly occurs at the subsequent third and fourth stage fluidized beds in series, and therefore the reduction efficiency of the fluidized bed reduction system decreases. Like the mainstream FIOR/FINMET and FINEX processes, the patents relating to reduction using four-stage fluidized beds also include: US20120328465 (2012), CN101892339 (2012), CN101397597 (2010), CN101519707 (2010), CN100560739 (2009), US20080277842 (2008), AU2001265669 (2001) and so on. In these processes, the reducing gas is operated in a serial mode, with high operating pressures and high consumptions for gas compression. Moreover, the reduction exhaust gas (generated in latter stages of fluidized beds) only utilize the sensible heat to preheat the mineral powder in the preceding stages of fluidized beds, with poor mineral powder preheating and reduction results.
In the Circore method (S. A. Elmquist, P. Weber, H. Eichberger, and Yuming Wang. World Steel, 2009, 2: 12-16) developed by Lurgi, Germany, hydrogen is used as a reducing agent. Iron ore powder and exhaust gas from the preheating fast bed enter into the fast fluidized bed with heat supply by direct fuel combustion through venturi and cyclone for drying and preheating to a temperature of 850-900° C., and are sent into the circulating fluidized bed (the first stage) at a temperature of 630-650° C. after being lifted into the ore bucket by the air for pre-reduction. Such preheating of iron ore powder by a fluidized bed is similar to those of the FIOR method and the FIMET method, except for relatively complex operations. The pre-reduced mineral powder discharged from the circulating bed is sent to the bubbling fluidized bed (the second stage) at a temperature of about 680° C. and a pressure of 0.4 MPa for final reduction. The high-temperature exhaust gas discharged from the circulating fluidized bed exchanges heat with the circulating gas, and is purified, compressed and recycled.
Lurgi also developed the Circofer method (U.S. Pat. No. 5,433,767; Shi Qiu, Sintering and Pelletizing, 1995, 2: 38-42) that uses coal as the main energy resource. In this method, iron ore powder, additives and hot exhaust gas discharged from the first stage fluidized bed (the circulating fluidized bed) are preheated by two stages of venturi preheaters. After the materials discharged from the first stage venturi preheater are separated by the cyclone, the resultant solid materials are sent into the second stage venturi preheater. After the materials discharged from the second stage venturi preheater are separated by the cyclone, the gas is sent to the first stage venturi preheater. During the above operation process, powder materials tend to recurrent accumulation in the preheating system. The solid materials discharged by separation through the second stage venturi preheater-cyclone are sent into the heat generator with heat supply by direct coal combustion for further preheating and generation of reducing gas, are re-sent into the first stage fluidized bed at a temperature of 950° C. for pre-reduction until the metallization rate reaches around 80%, and then enter into the second stage fluidized bed at a temperature of 850° C. for final reduction. The coal powder added to the heat generator can hardly undergo a sufficient reaction, and is discharged along with the directly reduced iron after passing through two stages of reduction fluidized beds. The residue coal powder subsequently needs to undergo magnetic separation to be recycled, thereby increasing the complexity of the operation.
The Outotec Company also proposed a fluidized bed direct reduction system based on a fluidized bed heated by combustion of carbonic material (the U.S. Pat. No. 7,608,128B2, and the Chinese invention patent ZL200580017740.5), which is similar to the Circofer method, and mainly consists of a heat-supply fluidized bed and a reduction fluidized bed. After being preheated by the hot flue gas discharged from the gas outlet of the cyclone separator of the reduction fluidized bed through two stages of mixed chambers-cyclones, ore enters into the reduction fluidized bed for reduction, which, however, is hardly preheated to the reduction temperature by the sensible heat of the reduction exhaust gas. The solid material discharged from the feed outlet of the cyclone separator of the reduction fluidized bed enters into the heat-supply fluidized bed. The coal powder added to the heat-supply fluidized bed is fluidized in the upward flow of the fluidizing (non-oxidative) air, and combusts to give a large amount of heat under the influence of oxygen-containing gas ejected downward from the water cooled lance installed in the heat-supply fluidized bed, which is provided to the reduction fluidized bed in the form of a dust-containing hot flue gas through the flue gas tube at the top of the heat-supply fluidized bed. The reducing fluidizing gas passes through the lower part of the reduction fluidized bed to realize the fluidized reduction of the mineral powder. Moreover, similar water-cooled oxygen-containing gas lance can be added in the reduction fluidized bed to realize effective control of fine particle agglomeration by introducing oxygen. The reduction exhaust gas after heat exchange with ore is recycled as the fluidizing gas after further solid stripping, cooling, dewatering, carbon dioxide removal, compression and reheating. This reduction system also has the problems of insufficient reaction of coal powder with complex operations for subsequent separation and recycling, and low efficiency in mineral powder preheating.
In addition to increasing system heat utilization and reducing energy consumption through technical innovations, the processes of iron ore concentrate reduction on fluidized beds still face the following two key problems: (1) High operating pressure. The existing multi-stage fluidized bed processes such as FIOR/FINMET and FINEX mainly employ gas serial operation and high pressure operation in order to reduce the diameter of the fluidized bed, thereby leading to high energy consumption during the air compression process. (2) Low reduction efficiency. Although most processes use four-stage fluidized beds for reduction, no intermediary heat supplementation for the gas passing from the fourth stage to the first stage fluidized beds in series results in gradual reduction of the reaction temperature from the fourth stage to the first stage. Since the last two stages play a minor role in the reduction, the overall reduction efficiency is low. Therefore, lowering operating pressure of the fluidized beds and increasing the overall reduction efficiency of the multi-stage fluidized beds through technological and technical innovations are crucial for improvement in economic efficiency of the reduction process.